2.4 INTESTINAL MICROFLORA AND ECOLOGY OF THE GUT The microflora of the human GIT is comprised of an extremely diverse microbial community encompassing both facultative anaerobic and obligate anaerobic microorganisms and is one of the preferred sources of potential probiotic micro- organisms destined for human use. While there are estimated to be more than 400 different species of bacteria in the gut, only 30 to 40 species represent 99% of the microflora found in any one human subject Throughout the GIT, the numbers and types of bacteria present exhibit significant variation right through from stomach to colon. Initially, bacterial cells arrive in the stomach washed with saliva from the oral cavity, and only the most acid-resistant microorganisms survive. Fewer than . cells per gram of mainly Streptococcus, Staphylococcus, and Lactobacillus species are found in the stomach Gram-negative and anaerobic bacteria predom-z inate in the distal ileum, reaching levels of 10 and 10' cells/ml of contents.20 In the colon, bacterial numbers reach levels of 10ll to 1012 cells/g of gut contents, with approximately 1014 bacterial cells present in the human colon, which is ten times greater than the tota number of cells in the human body.